In heavy earth moving equipment, such as draglines or power shovels for mining, an upper structure is rotatably supported on a lower structure by a roller circle assembly. A boom typically extends from the upper structure, and a sheave is placed on the upper end of the boom. A hoist rope extends generally downwardly from the sheave to a bucket or dipper, depending on the application. In the dragline application, the bucket is also connected to the upper structure by a drag rope. Both the hoist rope and the drag rope can control the movement of the bucket. The lower structure of the dragline typically is mounted on treads or a walking tub mechanism for movement over the ground. The roller circle assembly may include an upper circular rail coupled to the upper structure, a lower circular rail coupled to the lower structure, and rollers positioned between the circular rails. The rails can be in segments. The rollers are typically tapered, having their larger diameter located radially outward.
Vertical loads applied to the tapered rollers of the roller circle assembly induce thrust loads that tend to urge the rollers radially outwardly. These loads vary depending on the operation being performed, and can increase dramatically during digging operations. To counteract these radial forces, some roller circle assemblies are provided with rollers having flanges on their inner ends, commonly referred to as flanged rollers. The flange may bear against the inside edge(s) of the lower and/or upper circular rail(s), which thereby resist the tendency of the roller to move radially outward. In certain known bearing assemblies, contact between the flange and the circular rail can result in highly concentrated point or line contact that develops very high stresses. Moreover, when overturning moments associated with loads applied to the bucket or dipper are combined with rotation of the upper structure relative to the lower structure, the rollers within the roller circle assembly may become misaligned. Under certain loading scenarios, edges of the flange may contact the upper surface of the lower rail, or the lower surface of the upper rail. Such misalignment and undesired contact between the rollers and the rails can further exacerbate the point or line loading and result in additional significant stresses on the associated components. As a result, the contact surfaces, of the roller circle components, particularly the contact surfaces on the rail, may deteriorate due to spalling or chipping, which can eventually lead to failure of the roller circle assembly.
Roller circle assemblies in heavy earth moving equipment can measure from three meters to more than 20 meters in diameter. Very heavy loads, amounting to several hundred metric tons, may be transmitted through the bearing assemblies to the lower structure, and the overturning moments created by loads on the boom can create highly concentrated stresses on small segments of the bearing assembly. Moreover, the loading along a given arc length of the roller circle can be non-uniform during operations such as digging, dumping, transferring, and so forth. Under dynamic non-uniform loading, the rollers of the roller circle assemblies may twist or distort relative to the rails, further concentrating the load on a small contact area and bringing about costly failures.